a) Field of the Invention
This invention relates to a rolling bearing suitable for assembly, for example, in an automotive alternator having an aluminum-made housing or an electromagnetic clutch having an aluminum-made shaft so that the rolling bearing sis adapted to support a rotaing part thereof. It is to be noted that the term "aluminum as used herein means aluminum or an aluminum alloy.
b) Description of the Related Art
In a rolling part of an automotive alternator or of an electromagnetic clutch attached to a compressor for an automotive air conditioner, a rolling bearing, for example, such a rolling bearing as shown in FIGS. 1 and 2 is assembled. This rolling bearing is provided with an inner race 2 having an inner-race raceway 1 on an outer peripheral wall thereof, an outer race 4 having an outer-race raceway 3 on an inner peripheral wall thereof, and a plurality of rolling elements (5,5) interposed for rotation between the inner-race raceway 1 and the outer-race raceway 3. These plural rolling elements 5,5 are rotatably held by an annular retainer 6.
Between the outer peripheral wall of the inner race 2 and the inner peripheral wall of the outer race 4, bellow seals 7,7 are provided in a pair, one being at one end portions of the inner and outer races 2,3 and the other at opposite end portions thereof, whereby opposite openings of a space 8 defined between the inner-race raceway 1 and the outer-race raceway 3 are closed. A lubricant composition is filled between the paired seals 7,7 to ensure lubrication between the respective rolling elements 5,5 and the inner-race and outer-race raceways 1, 3.
When such a rolling bearing as described above is assembled in an alternator, the outer race 4 is fitted and fixed in an aluminum-made housing whereas the inner race 2 is fitted on a steel-made drive shaft. The aluminum-made housing however has low rigidity. When the alternator is driven, high load is applied to the housing under engine vibrations and belt tension so that the housing is prone to elastic deformation. As a result, some of the plural rolling elements 5,5 are caused to strongly contact the outer-race raceway 3, that is, so-called complex loading occurs. When the rolling bearing is assembled in an electromagnetic clutch attached to a compressor, the inner race 2 is fitted on an aluminum-made drive shaft of the compressor. When the compressor is driven, the aluminum-made drive shaft therefore undergoes elastic deformation. As a consequence, complex load is applied to the inner-race raceway 1 of the inner race 2. When such complex load is applied to the raceway surfaces (the inner-race race way 1 and the outer-race raceway 3) of bearing rings (the inner race 2 and the outer race 4), the raceway surfaces are susceptible to damage so that the life of the rolling bearing is shortened.
Accordingly, it has heretofore been the practice to make the inner race 2 and the outer race 4 with a bearing steel such as a high-carbon chromium bearing steel having an oxygen content of 9 ppm or so and then to subject raceway surfaces, which are susceptible to damage by the above-mentioned complex load, to standard heat treatment called "whole quench hardening", whereby the rolling contact fatigue strength of the raceway surfaces is improved. In addition, as the lubricant composition filled in the space 8, a lubricant composition comprising as a base oil a synthetic hydrocarbon lubricant such as a synthetic ester lubricant or poly-.alpha.-olefin lubricant having a kinematic viscosity of 30-50 cSt at 40.degree. C. and containing 13-17 wt. % of a thickener has been used to protect the raceway surfaces from damage.
Use conditions for a rolling bearing have however become more stringent, because the rolling bearing is subjected to greater load as a result of a higher temperature of a part where a rolling shaft is arranged, a higher revolution speed of the rolling shaft, and higher tension of a belt, all associated with the move toward higher performance of an automobile in recent years. The above-described conventional measure has therefore become insufficient to assure sufficient life in some instances, so that flaking or the like may take place at a relatively early stage on a raceway surface subjected to complex loading.
Keeping step with the move toward a high-performance automobile, an alternator is becoming smaller and its revolution speed is becoming faster. A rolling bearing, such as a ball bearing, assembled in such a small-size and high-rotational-speed alternator to support a rotating part therefore develops premature flaking which is accompanied by a characteristic change in texture totally different from conventional changes in texture. Such premature flaking is considered to take place because the rolling bearing is subjected to high load under engine vibrations and belt tension and, due to the low rigidity of a support portion of the rolling bearing, the races which make up the rolling bearing are repeatedly subjected to bending deformation.
With a view to prolonging the life of rolling bearings, some inventions have been made on the formulation of lubricant as disclosed in Japanese Patent Application Laid-Open (Kokai) Nos. HEI 1-259097, HEI 3-210394 and HEI 3-250094. Even if it is attempted to provide a rolling bearing with better lubrication by the grease compositions disclosed in these patent publication, it is still impossible to sufficiently prolong the life of the rolling bearing when the rolling bearing is under severe conditions. Further, French Patent Publication FR 2681655-A1 and German Patent Publication DE 4217566 A1 disclose inventions, in each of which it is attempted to prolong the life of a rolling bearing by imposing limitations on the kinematic viscosity of a base oil and the oxygen content of steel of races. In the case of the patents disclosed in these patent publications, however, no consideration is made with respect to a thickener so that the rolling bearings disclosed in these patent publications still cannot exhibit any sufficient advantage when they are used under severe conditions.